The British losses suffered in the attacks on the World Trade Centre will be greater than in any terrorist atrocity in history, it emerged yesterday.

Jack Straw, the foreign secretary, said that 100 Britons who were trapped in the twin towers were known to have died, and he warned that the final body count was "unlikely to be less than the middle hundreds and may be higher".

The death toll will easily eclipse Lockerbie, which claimed 44 British lives, and the Omagh bombing, in which 29 people died. Not since the second world war have so many Britons been killed in one day.

As harrowing stories of families still awaiting news of loved ones who are still missing, presumed dead, began to emerge, it became clear that their agony is likely to be drawn out for some days yet.

The process of recovering bodies from the tonnes of debris will last weeks and formally identifying some of the bodies will also be prolonged.

Until then, some families will cling on to the hope that their loved ones could still be found while others are already resigning themselves to the inevitable and beginning to mourn.

Another heartbreaking problem for many who are stuck abroad is that they are not able to get back to America to help in the search for relatives or even just to be near the scene of the disaster. The Foreign Office has yet to say whether it will help fund flights to the US for bereaved families.

The FO was under immense pressure to cope with a deluge of inquiries from people who had not heard from relatives or friends since Tuesday.

By yesterday evening, more than 14,000 people had contacted a helpline set up by the FO and partly staffed by trained police counsellors. Such has been the demand that the helpline is now running around the clock.

One of the most crucial and complex jobs of FO staff is to collate a database of Britons still missing and those not accounted for.

But because nationality is not listed on internal flight lists, staff are having to cross-refer names on passenger lists with names of people reported as missing. They are also feeding in information from the consulate-general in New York and the American emergency services.

A spokeswoman said: "The whole operation has been running pretty efficiently under the circumstances. It is very difficult for the US authorities to establish the full details of the casualties."

Consular officials in New York are appealing for all British citizens to make contact, whether or not they worked at the World Trade Centre, so that they can be accounted for.

It could be Sunday before names of those who lost their lives begin to be released. Identification itself is likely to be a lengthy process in some cases because of the severe injuries some of the deceased are bound to have.

A small team of police officers from Scotland Yard will be flying to New York to help the FBI and the New York police department deal with British fatalities. The officers will provide expertise in identifying the dead and will also try to look after the needs of families of British victims.

The Foreign Office has promised that the government will pay the hospital bills of any British citizens injured in the disaster and not covered by insurance and also meet the cost of flying bodies back.

The ministry would not discuss whether it would help towards the cost of flying bereaved families to America.

Britain's worst disasters

In 1966, a slag heap collapsed and engulfed a primary school in Aberfan, killing 144 people, including 116 children.

The sinking of the Titanic was a disaster on an enormous scale: 1,517 drowned when the liner sank, though not all were British.

However, the total scale of the World Trade Centre attacks compares more closely to wartime horrors than any peacetime tragedy.

In 1940, 4,500 British servicemen drowned when the Lancastria troop ship was sunk by German dive-bombers off the coast of France.

At least 40,000 were killed during the blitz, including 1,200 in Coventry and 20,000 in London.

Twenty thousand British soldiers were killed on the first day of the Somme in 1916.